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Old 09-07-2006, 09:45 AM   #1
BB68Vett
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Default Radiator Cap using overflow tank

I have '68 427/390hp. My car originally did not come with an external expansion tank or an external overflow/collector tank. Years and years ago I added an external overflow/collector tank but was never able to properly secure it. Recently, I got around to purchasing a different style overflow/collector tank and secured it to the inside of the right fender. I thought it would be a good idea at this point to replace my radiator cap but I want to make sure I am using the right one for use with an external overflow/collector tank.

The stock replacement cap as listed in the Stant application guide is either a #230 or #330 cap which is 16 lbs. Stant lists this as "for radiator overflow reservoir systems". A very similar cap #331 (also 16lbs) is listed also listed as "for radiator overflow reservoir systems" but also lists the cap as being "vented".

Do I need to purchase the vented cap to allow the coolant from the overlow tank to be drawn back into the radiator as it cools and the pressure drops? Will the stock replacement (non vented) cap serve the same purpose and allow coolant recovery back into the radiator? I am leaning toward the need for a vented cap to allow coolant recovery but can someone please confirm. Thanks.
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Old 09-07-2006, 06:52 PM   #2
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ttt Can anyone shed any light? Thanks.
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Old 09-07-2006, 07:03 PM   #3
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Quote:
Originally Posted by BB68Vett
I have '68 427/390hp. My car originally did not come with an external expansion tank or an external overflow/collector tank.
The main question that comes to mind is wether the 68' was a closed or open system?
If it diddnt come with the tank then I would guess it's not a closed system.
Can somebody else shed some light on this topic?
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Old 09-07-2006, 07:19 PM   #4
Jims79
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I have an overflow/refill tank on my 66. There is no filler on the radiator itself so I fill from the overflow tank. I use a Napa 14lb rad cap w/the flip top lid to release pressure. No problems so far.
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Old 09-07-2006, 08:09 PM   #5
paul 74
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If you have a rad cap on your rad you have an overflow tank. That cap on the rad is vented and not pressurized. If you do not have a cap on the rad, you have a expansion tank with a non-vented and pressurized cap. This one has no rad cap and is under pressure.

Click the image to open in full size.
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Old 09-07-2006, 08:49 PM   #6
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Quote:
Originally Posted by BB68Vett
I have '68 427/390hp. My car originally did not come with an external expansion tank or an external overflow/collector tank....
From that I assume you have a filler neck and radiator cap on the radiator.
The tank you added is an "overflow bottle" vs an expansion tank as the ones that came on cars with a radiator with no filler neck.
The overflow tank you has uses the overflow tube of the rad neck to expell excess coolant as it expands into the overflow bottle.
You can use either cap 330 or 331, with the non vented or full pressure cap being the more reliable. A V in the part number should mean vented. Here is why

The vented cap does not have a spring on the vent valve and the non vented has a light spring on the vent valve.

It has been found that the vented cap doesn't always close so you have a system that is not fully pressurized and can expell coolant and/boilover, increased pump cavitation and boiling hotspots in the head.

When using the none vented type make sure you have most of the air out of the system because it takes more heating/cooling cycles to purge air out of the system than does a vented cap.
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Old 09-07-2006, 09:20 PM   #7
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Quote:
Originally Posted by paul67
If you have a rad cap on your rad you have an overflow tank. That cap on the rad is vented and not pressurized. If you do not have a cap on the rad, you have a expansion tank with a non-vented and pressurized cap. This one has no rad cap and is under pressure.
Im sorry Paul but I do not understand your post at all
My older GM cars had a tube running off the fill on the rad that basically went to the ground
My newest vehicle is a 99 Explorer V8
It has a cap and an expansion tank as does my 73' Corvette.
Can you please elaborate because you have me scratching my head big time
Thanks
Bob
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Old 09-07-2006, 11:30 PM   #8
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Thanks for all the responses. I do appreciate it.

So according to Noonie, I can use either a vented or non-vented cap and either will allow recovery of the coolant back from the overflow bottle into the radiator as the engine cools. Have I misunderstood? I was under the impression that a non-vented cap would not allow recovery of the coolant from the overflow bottle back into the radiator. If I can use either vented or non-vented then what is the difference between a vented and non vented cap from a functional perspective.

By the way, Noonie is right on regarding the non-vented cap having a spring on the vent valve and the vented cap has no spring (and actually this is what raised a question in my mind as to whether I am using the right cap)

Just to clarify, I do not have an expansion tank plumbed into the heater hoses. I merely added a coolant overflow bottle so when my radiator expels coolant as the engine heats up it can be recovered back into the radiator as the engine cools. The overflow bottle just has a plastic cap on it, not a pressurized cap or anything like that.

I apologize if I am a little thick here.
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Old 09-07-2006, 11:53 PM   #9
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BB68vett - Same situation here, same engine (68 L-36). Just attach your overflow hose to the BOTTOM of the coolant overflow bottle. Actually, I replaced the overflow hose with a clear plastic hose of the same size. I didn't change the radiator cap and it works just fine. Truth is, it doesn't overflow any more, but when it did the coolant would get sucked back into the radiator when the engine cooled off.

The overflow bottle that I bought actually came with a "special replacement" radiator cap, but I never used it. Just kept the old radiator cap and it worked fine.


68 overflow hose dumps directly to the ground Even if your cooling system works great and "never" overheats you don't want to find yourself stuck in traffic on a 100 degree summer day and start dumping coolant on the street. Or pull into a parking lot and shut your engine off and start puking coolant in the parking lot. Good insurance to have an overflow bottle!

Last edited by PRNDL; 09-07-2006 at 11:57 PM.
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Old 09-08-2006, 12:00 AM   #10
BB68Vett
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So the stock replacement non-vented cap allows recovery of the coolant back into the radiator or does it just catch the coolant as the radiator expels it?
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Old 09-08-2006, 12:33 AM   #11
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Quote:
Originally Posted by BB68Vett
So the stock replacement non-vented cap allows recovery of the coolant back into the radiator or does it just catch the coolant as the radiator expels it?
First, I must commend you for your question on the differences. Most people just put on a cap that physically fits and that's it.

The original cap should not have the vent valve at all, so it cannot recover coolant back into the rad.

I use the 330. It has a lower failure rate of keeping the system fully pressurized than the "no spring" cap.
Oem's use the 230 because it purges the system much quicker and it is cheaper. Both the 230 and the 330 ultimately do the same job.

Just make sure you have it plumbed as PRNDL said. Some of the tanks don't have a bottom outlet and use a 2 hole cap with a vent and a long hose or tube to the bottom. Also make that hose is good if it applies.
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Old 09-08-2006, 12:35 AM   #12
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is there a certain # we are supposed to use? You guys are using #'s like 14,16,etc...but I just looked at my cap and it says 7....is that too low? how do I know what to use?
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Old 09-08-2006, 01:10 AM   #13
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The lower # ratings were used before all the emmissions and since they started using higher pressures hence the addition of the recovery bottle. An environmental thing.

I use the higher ones.
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Old 09-08-2006, 01:37 AM   #14
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Quote:
Originally Posted by Bob Onit
Im sorry Paul but I do not understand your post at all
My older GM cars had a tube running off the fill on the rad that basically went to the ground
My newest vehicle is a 99 Explorer V8
It has a cap and an expansion tank as does my 73' Corvette.
Can you please elaborate because you have me scratching my head big time
Thanks
Bob
Many Vettes came with a radiator that does not have a filler neck and therefore no place for a cap to fit, i.e. you can not fill the radiator with coolant by taking off a cap. The radiator top has only a thin tube to which a hose is attached. This runs to the expansion tank. The cap is on the expansion tank up by the right fender like in Paul's picture. That's where coolant is added. It enters the engine through the heater hose lines, although some of it is probably drawn back into the radiator through the expansion hose as the fluid cools.

This link should show a picture of the radiator. Notice the small tube at the top for the expansion.
Click the image to open in full size.
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Old 09-08-2006, 01:43 AM   #15
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Quote:
Originally Posted by BB68Vett
So the stock replacement non-vented cap allows recovery of the coolant back into the radiator or does it just catch the coolant as the radiator expels it?
It drains back to the engine through the heater hoses on the earlier C3s.

Click the image to open in full size.
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Old 09-08-2006, 03:06 AM   #16
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Quote:
Originally Posted by Z-man
Many Vettes came with a radiator that does not have a filler neck
Gotcha!
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Old 09-08-2006, 06:34 AM   #17
paul 74
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Bob,
I should have been more precise in my language. What I was trying to say is that I do not have a cap on my rad. The pressurized cap (15psi) is on the expansion tank.

Click the image to open in full size.

Click the image to open in full size.

This is the overflow tank on my 1979. The rad does have a pressurized cap and the tank is not under pressure. Basically it just accepts hot coolant that in earlier times used to flow straight to the ground.

Click the image to open in full size.

Last edited by paul 74; 09-08-2006 at 06:45 AM.
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Old 09-08-2006, 08:24 AM   #18
BB68Vett
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Paul67, you are up early in the morning.

I have installed a system just like your 79. I want to make sure that in addition to the overflow bottle catching the expelled coolant, that the radiator cap will allow the coolant to be re-introduced into the radiator as the engine cools.

So what I am trying to figure out is whether I need a vented cap to accomplish this. If either a vented or non-vented cap will accomplish it then what is the functional difference between the two different types of radiator caps.

BTW, thanks to all for the replies so far.
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Old 09-08-2006, 10:11 AM   #19
paul 74
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The words "vented" and "non-vented" confused me but after checking the Stant web site I think I have things sorted out. But people will pipe up if I am wrong.

My conclusion is that either one can be used. Both have a return valve/seal to allow coolant flow back to the rad when cooling occurs and negative pressure develops. One cap is sealed (non-vented) and must be opened to relieve pressure. The other has a vent (see red handle in my pic above) to relieve pressure without removing the cap. I have used both on my 1967 although in that system there is no return valve/seal. This may be a simplistic and erroneous conclusion but we shall see.
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Old 09-08-2006, 11:21 AM   #20
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Folks - we need to watch it here. Paul will use any excuse to take more pictures and show off his beautiful engine compartment...
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Old 09-08-2006, 11:21 AM
 
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